In a typical axial flow gas turbine, hot, high pressure working fluid comprising air and products of combustion is transmitted into a turbine nozzle structure which is usually annular in shape. The working fluid accelerates through the nozzle structure in a direction designed to thermodynamically optimize its subsequent engagement with blades mounted on the turbine's rotatable rotor. The turbine nozzle structure, accordingly, is subjected to large pressure loads due to the reduction in static pressure of the working fluid during its acceleration and differential thermal expansion loads resulting from relatively low working fluid temperatures at the radial inner and outer margins of the nozzle structure and relatively high working fluid temperatures intermediate such radial margins. Such turbine nozzle structures have typically been geometrically positioned in their desired location by clamping same between axially adjacent faces of mounting structure.
In the quest for increasing turbine efficiency, working fluid temperature increases have been sought as well as structure to accommodate same. Ceramic nozzle structures have become increasingly favored due to their ability to function satisfactorily in high temperature environments. Ceramic nozzle structures are, however, typically mounted on metallic supporting structures which commonly constitute the majority of structural members in gas turbines. Differential thermal expansion between ceramic nozzle structures and the metallic supporting structures therefor and the resulting high thermal stresses therein virtually prohibit the use of the aforementioned clamping nozzle support structure.
Very recently, however, the assignee of the present invention developed a cantilevered ceramic nozzle structure employing a radially outer shroud having airfoil vanes connected at one end thereto and protruding radially inwardly therefrom and a radially inner shroud which is radially spaced from the free ends of the airfoil vanes.
While such cantilevered nozzle structure substantially reduces the stress induced in nozzle structures by differential thermal expansion as compared to that experienced by conventional nozzle structure components, mounting same to a metallic support structures typically used in today's gas turbines exacerbates the problems encountered in resisting pressure reduced loads thereon since those loads must be reacted entirely through the outer shroud while precisely positioning the connected airfoil vanes in the hot working fluid flow path.
Pins and axial oriented fasteners have frequently been used to mount and fix componentry within gas turbines. German patent 1,035,662, which issued Aug.7, 1958, used axial pins to join a covering to the outer ends of the rotatable blades in a turbine. U.K. patent 532,372, having a convention date of Aug.27, 1938, employed pins for fixing arcuately adjacent, rotatable turbine blades to each other. U.S. Pat. No. 4,815,933, which issued Mar. 28, 1989, used pins for connecting conventional turbine nozzles to nozzle supporting seats. The following U.S. patents used pins to affix turbine nozzles of conventional, integral dual shroud/airfoil vane construction to nozzle support structures: U.S. Pat. Nos 4,883,405, which issued Nov.28, 1989; 3,363,416, which issued Jan. 6, 1968; and 5,211,536, which issued May 18, 1993.
To successfully use the cantilevered nozzle structure for accelerating high temperature working fluid therethrough, the nozzle support structure must provide a fixed clearance between arcuately adjacent nozzle segments, a precise axial and radial location for nozzle segments, and a relatively loose attachment joint for frictionally damping certain modes of airfoil vane vibration.